8.5 Greenhouse effect (anirudh) 3 hours The Anirudh Overview Fundamentally, the Greenhouse effect results from the fact that the Earth reflects into space only 30% of the solar radiation it receives, whereas 70% is 'trapped' (as in a Greenhouse) as gases in the atmosphere absorb the heat and re-emit it towards the Earth's surface, causing a warming of the land, atmosphere, and ocean.
The writing in RED on this page is the teacher notes that appear in the IB syllabus.
8.5.1 Calculate the intensity of the Sun’s radiation incident on a planet.
The key thing to remember is that most of incident solar radiation is in the visible light wavelength range, or a 'near visible' wavelength range that includes infrared and UV radiation.
Incident Solar intensity may therefore be estimated using an Intensity-Wavelength spectrum as below:
Image source: Wikimedia commons
Here we have that the Sun emits most of its radiation between 0.1 and 4.0 micrometres, which is a visible light and 'near visible' wavelength. We can therefore make assumptions about this 'near visible' nature of solar radiation when estimating the intensity.
8.5.2 Define albedo.
The albedo of an object is a measure of that objects extent to which it diffusely reflects light from the sun.
It is stated as a ratio of (1) diffusively reflected to (2) Incident EM radiation. It therefore has no units.
The connection with the greenhouse effect should be intuitively clear.
8.5.3 State factors that determine a planet’s albedo.
Students should know that the Earth’s albedo varies daily and is dependent on season (cloud formations) and latitude. Oceans have a low value by snow a high value. The global annual mean albedo is 0.3 (30%) on Earth.
Besides the useful 'teachers notes' in red above, which is probably all that you need to know, these factors also affect Albedo:
I have already written a little bit about it in 'The Anirudh Overview'.
The radiation comes in visible radiation with short wavelength, most is absorbed by the earth's surface, and the rest is re-emitted as Infrared Radiation however some of this radiation is captured by the earth's atmosphere and then reflected back to the earth's surface as heat.
8.5.5 Identify the main greenhouse gases and their sources.
The gases to be considered are CH4, H2O, CO2 and N2O. It is sufficient for students to know that each has natural and man-made origins.
Remember that these gases absorb and emit radiation in the infrared wavelength range.
Now, here are the depressing trends in greenhouse gases:
Image source: Wikimedia commons
8.5.6 Explain the molecular mechanisms by which greenhouse gases absorb infrared radiation.
Students should be aware of the role played by resonance. The natural frequency of oscillation of the molecules of greenhouse gases is in the infrared region.
Infrared raditation causes the atoms to vibrate in different ways. Resonance occurs when the frequency of the infrared radiation and the frequency of vibration are equal. As implied in red, only those molecules which vibrate at the same energy as photons of infrared light (when resonance occurs) can be greenhouse gases. However, molecules such as HCl or CO, while they absorb IR, are short lived due to their reactivity and solubility. Thus, to have a natural frequency in the infrared region is a prerequisite for being a greenhouse gas, but does not validate it.
Remember that the two most abundant gases, N2 and O2, are not greenhouse gases because of this reason.
8.5.7 Analyse absorption graphs to compare the relative effects of different greenhouse gases.
Students should be familiar with, but will not be expected to remember, specific details of graphs showing infrared transmittance through a gas.
Please read the green book for this objective
8.5.8 Outline the nature of black body radiation
Students should know that black-body radiation is the radiation emitted by a “perfect” emitter.
I once touched a black tennis racket cover that had been out in the sun for a while. It was hot.
In general, a black body in physics is one that absorbs all EM light that falls on it. It is a theoretical body, just like how an ideal gas is a theoretical gas (i.e. a black tennis racket cover is not really a black body). It is also a perfect emitter of radiation. Since it emits all wavelengths of radiation, the maximum wavelength of radiation for emission/absorbtion is infinite. However, black bodies do emit a maximum energy at a peak wavelength. Observe the following curve for a black body at 5000K:
As an excercise, draw the vertical line that would help you find that peak wavelength. Its not hard.
Again, bear in mind that the curve never actually touches the x-axis (but it does converge towards the x-axis).
8.5.9 Draw and annotate a graph of the emission spectra of black bodies at different temperatures.
Easy:
Image source: Same as before.
What a beautifully annotated graph we have here. Be able to draw this. I hope you've also picked up the following points, which I quote directly from the website from which I found these pictures:
1. As the temperature increases, the peak wavelength emitted by the black body decreases.
2. As temperature increases, the total energy emitted increases, because the total area under the curve increases.
Point (1) is known mathematically as Wien's Law: (from Wikipedia)
where b is just a constant in the data booklet.
8.5.10 State the Stefan-Boltzmann law and apply it to compare the emission rates of different surfaces. For a black body, we have the following relationship;
P = Power (J/s) (Sigma) = Constatn in data booklet A = Surface area of black body (m^2) T = Temperature (K)
The emission rate here is power, since power = energy/time. So you have to basically know things like higher area results in a greater emission rate, and a doubling of temperature results in a sixteen-fold increase in emission rate.
8.5.11 Apply the concept of emissivity to the compare the emission rates from the different surfaces.
Emissivity is a ratio of (1) energy radiated by a specific material to (2) energy radiated by a theoretical black body. With our knowledge of black bodies, we know that (1) < (2). Thus emissivity is a value between 0 and 1. When we discuss emissivities of greenhouse gases, we qualify (1) as to energy radiated in the infrared wavelength range.
8.5.12 Definesurface heat capacity Cs.Surface heat capacity is the energy required to raise the temperature of unit area of a planet’s surface by one degree. Measured in J m-2 K-1
Each surafce has a different surface heat capacity and thus we use this equation to calculate the effective heat capacity:
Cs= fpch
where f=0.7 (fraction of earth covered by water)
p= the density of the sea water
c=the specific heat capacity of water
h=the depth of seawater that stores thermal energy
8.5.13 Solve problems on the greenhouse effect and the heating of planets using a simple energy balance climate model.
Students should appreciate that the change of a planet’s temperature over a period of time is given by : (incoming radiation intensity – outgoing radiation intensity) x time/ surface heat capacity.
Yeah, you need to read the green book and do the calculations. Good luck!
Students should be aware of limitations of the model and suggest how it may be improved.
3 hours
The Anirudh Overview
Fundamentally, the Greenhouse effect results from the fact that the Earth reflects into space only 30% of the solar radiation it receives, whereas 70% is 'trapped' (as in a Greenhouse) as gases in the atmosphere absorb the heat and re-emit it towards the Earth's surface, causing a warming of the land, atmosphere, and ocean.
The writing in RED on this page is the teacher notes that appear in the IB syllabus.
8.5.1 Calculate the intensity of the Sun’s radiation incident on a planet.
The key thing to remember is that most of incident solar radiation is in the visible light wavelength range, or a 'near visible' wavelength range that includes infrared and UV radiation.
Incident Solar intensity may therefore be estimated using an Intensity-Wavelength spectrum as below:
Image source: Wikimedia commons
Image source: http://www.physicalgeography.net/fundamentals/6f.html
Here we have that the Sun emits most of its radiation between 0.1 and 4.0 micrometres, which is a visible light and 'near visible' wavelength. We can therefore make assumptions about this 'near visible' nature of solar radiation when estimating the intensity.
8.5.2 Define albedo.
The albedo of an object is a measure of that objects extent to which it diffusely reflects light from the sun.
It is stated as a ratio of (1) diffusively reflected to (2) Incident EM radiation. It therefore has no units.
The connection with the greenhouse effect should be intuitively clear.
8.5.3 State factors that determine a planet’s albedo.
Students should know that the Earth’s albedo varies daily and is dependent on season (cloud formations) and latitude. Oceans have a low value by snow a high value. The global annual mean albedo is 0.3 (30%) on Earth.
Besides the useful 'teachers notes' in red above, which is probably all that you need to know, these factors also affect Albedo:
Meddle around with their effects here: http://snowdog.larc.nasa.gov/jin/albedofind.html
8.5.4 Describe the greenhouse effect
I have already written a little bit about it in 'The Anirudh Overview'.
The radiation comes in visible radiation with short wavelength, most is absorbed by the earth's surface, and the rest is re-emitted as Infrared Radiation however some of this radiation is captured by the earth's atmosphere and then reflected back to the earth's surface as heat.
This is really worth watching to grasp the fundamentals: http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/
8.5.5 Identify the main greenhouse gases and their sources.
The gases to be considered are CH4, H2O, CO2 and N2O. It is sufficient for students to know that each has natural and man-made origins.
Remember that these gases absorb and emit radiation in the infrared wavelength range.
Now, here are the depressing trends in greenhouse gases:
Image source: Wikimedia commons
8.5.6 Explain the molecular mechanisms by which greenhouse gases absorb infrared radiation.
Students should be aware of the role played by resonance. The natural frequency of oscillation of the molecules of greenhouse gases is in the infrared region.
Infrared raditation causes the atoms to vibrate in different ways. Resonance occurs when the frequency of the infrared radiation and the frequency of vibration are equal. As implied in red, only those molecules which vibrate at the same energy as photons of infrared light (when resonance occurs) can be greenhouse gases. However, molecules such as HCl or CO, while they absorb IR, are short lived due to their reactivity and solubility. Thus, to have a natural frequency in the infrared region is a prerequisite for being a greenhouse gas, but does not validate it.
Remember that the two most abundant gases, N2 and O2, are not greenhouse gases because of this reason.
8.5.7 Analyse absorption graphs to compare the relative effects of different greenhouse gases.
Students should be familiar with, but will not be expected to remember, specific details of graphs showing infrared transmittance through a gas.Please read the green book for this objective
8.5.8 Outline the nature of black body radiation
Students should know that black-body radiation is the radiation emitted by a “perfect” emitter.I once touched a black tennis racket cover that had been out in the sun for a while. It was hot.
In general, a black body in physics is one that absorbs all EM light that falls on it. It is a theoretical body, just like how an ideal gas is a theoretical gas (i.e. a black tennis racket cover is not really a black body). It is also a perfect emitter of radiation. Since it emits all wavelengths of radiation, the maximum wavelength of radiation for emission/absorbtion is infinite. However, black bodies do emit a maximum energy at a peak wavelength. Observe the following curve for a black body at 5000K:
Image source:http://www.egglescliffe.org.uk/physics/astronomy/blackbody/bbody.html
As an excercise, draw the vertical line that would help you find that peak wavelength. Its not hard.
Again, bear in mind that the curve never actually touches the x-axis (but it does converge towards the x-axis).
8.5.9 Draw and annotate a graph of the emission spectra of black bodies at different temperatures.
Easy:
Image source: Same as before.
What a beautifully annotated graph we have here. Be able to draw this. I hope you've also picked up the following points, which I quote directly from the website from which I found these pictures:
1. As the temperature increases, the peak wavelength emitted by the black body decreases.
2. As temperature increases, the total energy emitted increases, because the total area under the curve increases.
Point (1) is known mathematically as Wien's Law:
where b is just a constant in the data booklet.
8.5.10 State the Stefan-Boltzmann law and apply it to compare the emission rates of different surfaces. For a black body, we have the following relationship;
P = Power (J/s) (Sigma) = Constatn in data booklet A = Surface area of black body (m^2) T = Temperature (K)
The emission rate here is power, since power = energy/time. So you have to basically know things like higher area results in a greater emission rate, and a doubling of temperature results in a sixteen-fold increase in emission rate.
8.5.11 Apply the concept of emissivity to the compare the emission rates from the different surfaces.
Emissivity is a ratio of (1) energy radiated by a specific material to (2) energy radiated by a theoretical black body. With our knowledge of black bodies, we know that (1) < (2). Thus emissivity is a value between 0 and 1. When we discuss emissivities of greenhouse gases, we qualify (1) as to energy radiated in the infrared wavelength range.
8.5.12 Define surface heat capacity Cs. Surface heat capacity is the energy required to raise the temperature of unit area of a planet’s surface by one degree. Measured in J m-2 K-1
Each surafce has a different surface heat capacity and thus we use this equation to calculate the effective heat capacity:
Cs= fpch
where f=0.7 (fraction of earth covered by water)
p= the density of the sea water
c=the specific heat capacity of water
h=the depth of seawater that stores thermal energy
8.5.13 Solve problems on the greenhouse effect and the heating of planets using a simple energy balance climate model.
Students should appreciate that the change of a planet’s temperature over a period of time is given by : (incoming radiation intensity – outgoing radiation intensity) x time/ surface heat capacity.
Yeah, you need to read the green book and do the calculations. Good luck!
Students should be aware of limitations of the model and suggest how it may be improved.